Many methods are known and used in liquid filling machines for the purpose of placing a liquid into a container, or package. These include volumetric filling methods which define a liquid dose using a positive displacement pump such as a piston pump or using a rotary pump, or a timed flow of liquid at a defined flow rate.
It is well known that many packaged liquid products vary in density from batch to batch and also over time within a batch. Therefore, a fill dose which is volumetric will vary in its weight as the density of the liquid changes, even when the volumetric quantity of liquid produced by the filling machine does not change. In most liquid filling manufacturing operations, it is necessary to maintain the quantity of product being placed into the package within certain required limits. The quantity of product being placed into the package is almost always measured and monitored by weight, using a scale device. When a volumetric filler is used to fill a liquid which does not change in density, periodically measuring the net weight of the fill dose is a means of tracking the stability and reproducibility of the volumetric fill produced by the filling machine and adjusting it as necessary. When a volumetric filler is used to fill liquid products which change in density, it is necessary to adjust the fill volume to follow the changing fill weights. In either case, if the filler used weight to determine the dose, the problems of tracking and adjustment of liquid fill dose would be solved.
Various means to fill liquids by weight are known. However, each has disadvantages and shortcomings. In one method, a volumetric fill is produced which is separately checked by weight using an electronic scale. The net weight obtained by checking is then used as a feedback signal to the filler to automatically adjust the volumetric dose to maintain a uniform liquid fill weight with varying machine or product conditions. The primary problem with this method is that in order to obtain a liquid weight on a scale with sufficient accuracy to be useful to adjust a filler, generally to better than one half of one percent of the nominal fill weight, requires a period of time that is too long to allow rapid production of filled containers. This is particularly the case because liquids tend to remain in motion within a vessel for a prolonged period, thus perturbing the scale readings. In addition, to determine the net weight of liquid being weighed on the scale, the weight of the container into which it has been filled must be know. This requires a pre-weighing or tare of each container before filling, using the same or a separate scale. This requirement further complicates and slows the process. In addition, in an automated system, two scales are required, one to tare the empty container and one to weigh the filled container, and this leads to very high expense which is rarely justified by the slow output speed of filled containers possible from such an arrangement. It is possible to disregard the container weight. However, it is well known that most containers vary significantly in weight one from the next and many vary to such a great degree as to not allow repeatable liquid filling within the required degree of accuracy.
In another method, liquid filling is done in such a way that the container is placed upon a scale and liquid is dispensed into the container until the desired net weight is reached. This is referred to as filling by weight or net weight filling. The problem with this method is twofold. First, the turbulence and relative motion of the liquid makes getting a precise weight difficult as a function of filling speed (flow rate). Second, electronic scales are limited in the rate at which they can track a dynamically changing weight. This limits the rate at which liquid can flow into containers. Further, before filling can begin, the container must be tared to eliminate its weight. This requires significant time as well.
In some designs, the liquid is introduced at a comparatively high flow rate until nearly all of the fill weight has entered the container, and then the flow rate is reduced to a very low flow rate to approach the desired weight with more precision. This is known as the "bulk and dribble" technique and it has the effect of slowing down the filling process as well.
In automated systems, either method is typically confined to expensive pharmaceutical fillers operating with a dedicated or narrow range of liquid products and fill weights at relatively low speeds, or to expensive rotary net weight fillers which operate at high speeds by virtue of having many scale filling positions, and which typically fill a narrow range of container sizes and weights. Another problem with scale based net weight filling is that scales cannot, in commercially practical terms, provide the dynamic range of weighing capability needed to allow their use in automated filling machines where a very large range of fill sizes must be filled on the same liquid filling machine. For example, a rotary pump volumetric filler could fill 100 mL containers with .+-.0.5% repeatability on the one hand, and then could fill 10,000 mL containers at .+-.0.5% repeatability on the other, without any change in the fitments or features or apparatus of the machine. This range of capability cannot be achieved with a scale based net weight filling machine without the need for substantial changes to the machine. Finally, scales impose very severe restrictions upon the means of construction of the machine itself, particularly where such a machine is to be constructed as an in-line (as opposed to rotary) intermittent motion automatic liquid filler. For example, because each container to be filled must be isolated from any other container, the containers must be properly spaced and separated upon the machine, a task which is completely unnecessary in volumetric machines. Furthermore, each scale must be mechanically isolated from the machine to prevent vibrations from affecting its operation and accuracy. In addition, it is impractical to add scale filling positions to scale based in-line machines as a means of increasing machine capacity.